The question is: At the end of 2007 the company accrued salaries of $45 000 in excess of the correct amount.

Prepare the journal entry in 2008 to correct the books.

The answer is:
Sales Salaries Expense 45,000
Retained Earnings 45,000

I don't understand why... what is the closing entries again? I forgot.

THanks in advance.

Closing entries are journal entries at the end of a period to transfer the net effects of revenue and expense items from the income statement to owner's equity.
Your expense accounts are credited and your income summary account is debited.
Next your revenue accounts are debited and your income summary account is credited.
Next you close out you're your income summary account to retained earnings.

This is done so that your temporary accounts (Income Statement Accounts) will have Zero balances at the start the new accounting period. The amounts are ultimately transferred to your permanent accounts (Balance Sheet accounts).